Power switching assemblies have many uses, including converting AC current to DC current and converting DC current to AC current. DC current may be converted to AC current for the purpose of driving an AC motor or as part of a process for converting DC current at a first frequency and/or voltage to DC current at a second frequency and/or voltage. These conversions and mechanisms for accomplishing these conversions are frequently necessary in the electrical engineering world. As fossil fuels become rarer, it is likely that the need to perform these conversions will become even more common, as electrical energy is gleaned from nature-dictated, rarely tapped sources, such as sun, wind and ocean movement, and processed into forms that electrical appliances have been built to accept.
A few fundamental problems face designers of power switching assemblies. Whereas the transistors, typically metal oxide semiconductor field effect transistors (MOSFETS) or insulated gate bipolar transistors (IGBTS) are made largely of silicon oxide crystal, which has a coefficient of thermal expansion (COTE) of about 2.3. Although it is advantageous to use a high thermal conductivity material to draw the heat out of the transistors, only diamond has both high thermal conductivity and a low COTE. Accordingly, most high thermal conductivity materials used create a COTE mismatch at some point in the system.
In addition, both MOSFETs and IGBTs have a very thin (@100 nm), very delicate layer of metal oxide on a major surface of the silicon oxide substrate. Great care must be taken in electrically connecting this layer, to avoid a connection that would physically damage the metal oxide layer during manufacturing or thermal cycling.
In a separate development, due to an ability to withstand higher temperatures than other popular semiconductors used for power switches, silicon carbide metal semiconductor field effect transistors (MESFETS) are gaining popularity in the power semiconductor field.